Page 143 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 119
The true flame temperatures are much lower than the values calculated in
this way, because combustion is far from adiabatic.
4.2.7. Dissociation energy, bond energy and enthalpies of
formation
A second way of determining the enthalpies of formation is to use the
bond energies.

4.2.7.1. Enthalpy of formation from gaseous monatomic elements
Up until now, we have considered the energies of formation from the
simple substances taken in their normal state in standard conditions. We
shall now turn our attention to the enthalpies of formation of the compounds
from their constituent atoms in the gaseous state, which we call the enthalpy
of synthesis. We also find the opposite of that value: the enthalpy of
atomization, which is the enthalpy released during the complete separation
of a substance into single atoms. In order to switch from our standard
enthalpies of formation to these enthalpies of synthesis, we need to know the
enthalpies of dissociation of the simple substances and the enthalpies of
sublimation and vaporization for the solid and liquid monatomic compounds.

4.2.7.2. Energy of sublimation
Thanks to experimental determination, we know the enthalpies of
sublimation of numerous solids at ordinary temperature, such as sulfur,
selenium, phosphorus, tin, etc. The only truly tricky point, in fact, is
determining the enthalpy of sublimation of carbon in graphite (or diamond)
form, about which there have been a great many studies conducted. We
cannot be certain of the values that are currently accepted.
4.2.7.3. Energy of dissociation

Consider the water molecule in the gaseous state. It comprises two
OH bonds.

Let us consider the breaking of the first bond by the following reaction:

H 2O(gas) = H(gas) + OH(gas) [4R.2]